A detector is built on a semiconductor wafer of the type in which a concentrated radiation causes charge carrier pairs (holes and electrons) to form in the irradiated point.
The semiconductor wafer is provided with conductive layers, one of which also serves as a barrier layer, and whose extension coincides with the active surface, with the opposite conductivity type to that of the semiconductor wafer material. This layer prevents the charge carrier pairs formed in connection with the irradiation from recombining inside the semiconductor wafer. Instead, the recombination occurs outside the wafer by the two conductive layers being connected to an external electric circuit. This will cause the charge carriers to move through this connection, thus producing an electric current.
The position determination is obtained by the charge carriers being distributed, by means of a resistive splitting-up of the current, among collecting electrodes arranged on the conductive layers. By measuring the currents in the outer connections between the electrodes of the respective barrier layers, the position of the point of light may be determined.
Depending on the number of electrodes on the conductive layers and their arrangement, the detector can be considered to have a number of axes, along which the current is divided. In the case where two electrodes are provided, the detector is considered to be a detector with one axis, and in the case of four electrodes it is considered to be a detector with two axes etc. For example, a detector where one conductive layer may have four electrodes between which the current is divided, while the other layer has only one collecting electrode would be a detector with two axes. The same number of axes can also be obtained if the two conductive layers have two electrodes each.
The conductive layers and the electrodes may be designed in any optional way, which means that the axis along which the current is divided does not need to be of a particular appearance and does not necessarily have to be a straight line.
A device of this type is known from Applicant's Patent Application EP 0 801 725, with the same assignee. A disadvantage of such previously known devices is, however, that the sensitivity, i.e. how much current that is generated by the point of light and that can be divided between the electrodes, is limited by the number of charge carrier pairs that are being generated for each incident radiation quanta. This number is limited by the laws of physics. The consequence of this is that a detector of the above type cannot be used in very many applications or that the result is not entirely reliable. Situations where these previously known detectors have proven inadequate are, for example, in measurements carried out over long distances or measurements where reflections from surfaces with low reflectivity are being used.